The invention relates to a process and a device for cold milling in a fluidized-bed counter-jet mill.
Jet mills are pulverizing machines which have been known for a long time and in which the particles to be milled are accelerated by gas flows and pulverized by collision. There are a number of different types of jet mill designs. They differ in the technique of the gas flow, in the way that the particles collide against each other or against a collision surface, and with respect to whether the particles to be milled are carried along in the gas flow or whether the gas flow strikes the particles and sweeps them along. Air or hot steam are generally used as the milling gas.
In the case of the fluidized-bed counter-jet mill, freely expanding jets of gas meet each other in a milling chamber containing the material to be milled in the form of a fluidized-bed. Here, the milling occurs practically exclusively as the result of the collision of the particles of material against each other; hence, the milling process is virtually wear-free. The fluidized-bed counter-jet mill has a grader in which the finely milled material is separated from larger particles that have not yet been sufficiently pulverized. The large pieces are returned to the milling chamber.
Many substances, for example, plastic, are difficult or impossible to mill to a fine grain size because of their toughness. However, when such tough materials are exposed to cold, they become brittle, which improves their milling properties. Therefore, the propellant gas is cooled when jet mills are used; this is described, for example, in West German laid-open application No. 2,133,019. Cooling the propellant gas flow makes it possible to mill materials that could not be milled under normal conditions in jet mills. However, in spite of intensive cooling, for example, with liquid nitrogen, and in spite of the self-cooling effect of the propellant gas flow as a result of its expansion, the attainable improvement of the milling properties leaves a great deal to be desired. Although fine grain sizes can be achieved, this is only possible at an excessively high consumption of time and energy.